Rooster losing his voice?

My rooster has lately been losing his voice it seems like. Its been getting worst, i've noticed it about a few weeks ago. When he crows you can barely hear it. I've been giving him so tylan because its known to get rid of respiratory problems in poultry, but that doesn't seem to be doing anything either. I've checked if it was wet pox and it wasnt that. I checked if it was canker and it wasn't that either. I was thinking it was the new bedding i got him because before he just had dirt in his coop. but i've noticed that before i put the bedding in he was already starting to lose his voice. So what can it be? Any help would be appreciated! thanks.